It’s been more than three years since Russia's sweeping effort to interfere in U.S. elections through disinformation on social media, stolen campaign emails and attacks on voting systems.

U.S. officials have made advances in trying to prevent similar attacks from undermining the 2020 vote, but challenges remain.

U.S. intelligence chiefs say potential threats aren’t just from Russia, but also from China, Iran and North Korea.

And there have been indications that hostile foreign governments might disguise their attacks to make them appear to be caused by other hackers.

Campaigns and state and local election systems remain attractive targets.

Social media is still a convenient method for sowing division.

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